The present invention relates generally rotary printing apparatus and methods and, more particularly, to operations in which a series of minute dots are transferred from a rotary printing member to a substrate in order to produce vignettes or other printed subject matter.
The present invention generally involves two types of printing operations. In the first type, a rotating printing member transfers a series of minute dots with a gradually changing density onto a substrate until they fade into the background color, which may be white or another color. This particular type of printed substrate is referred to as a vignette. FIG. 1 shows a typical printing apparatus 10, which may be a flexographic printing press, including a print cylinder 12 with a flexographic printing plate 14. An impression cylinder 16 forces a substrate, such as a web of material 18, against print cylinder 12 as the print cylinder 12 and impression cylinder 16 rotate and the web 18 translates therebetween in a linear direction. In this manner, a vignette 20 may be produced on substrate 18.
As shown in FIG. 1, vignette 20 is particularly susceptible to visible imperfections known in the industry as barring, banding, streaking or gear marking. Specifically, one or more darkened bars or bands 22 can materialize on the vignette 20 in a direction perpendicular to the machine direction or, in other words, the direction of the substrate 18 moving past the rotating print cylinder. Often, this barring or banding phenomenon occurs for unexplainable reasons. The human eye perceives these bars or bands much more readily in vignettes as the color gradually transitions from a darker region to a fully faded region. The most problematic area of a vignette tends to be between about 15% and 70% dot coverage or density.
As further shown in FIG. 1A, typical dots used to produce single color vignettes are round. A bar or band may be produced when the machine components of the printing apparatus 10 cause even a very slight difference in speed between the rotating print cylinder 12 and the moving substrate 18. Essentially, this causes a very slight skidding effect between the substrate 18 and the print cylinder 12 which causes dots 24 to deform into dots 26 in region 22. Thus, dots 24 have the intended diameter D1, while dots 26 have a diameter D2 which is increased in the machine direction 28. The sporadic, relative movement between print cylinder 12 and substrate 18 is typically less than 0.001 inch. Many in the industry believe that the barring or banding effect arises due to the imperfect tolerances between moving parts of the printing press, such as in the gearing which drives the various cylinders. However, numerous attempts to improve the mechanics of a printing press have failed to prevent the barring or banding phenomenon. Moreover, barring or banding arises very often in a sporadic and unpredictable fashion making the true cause of these imperfections very illusive.
Other attempts to reduce the amount of banding or barring involve the use of particular types of substrates. For example, an article entitled Final Report: Narrow Web Flexographic Banding in the August 1996 issue of Flexo magazine suggests that rough, absorbent substrates should be used to decrease the instances of barring or banding. However, as mentioned in the article, these substrates tend to be lower quality substrates. Also, while the use of a particular type of substrate may or may not prevent or reduce the visibility of these imperfections, changing the type of substrate is simply not possible for many applications.
The other type of printing to which the present invention is directed is referred to as process printing. In process printing, multiple series of dots are laid down on a substrate one color at a time to produce a pattern and ultimate color desired for a particular application. In one conventional type of process printing, referred to as a four color process, four print cylinders may be used to successively lay down the colors yellow, magenta, cyan and black. To prevent so-called moirxc3xa9 patterns from becoming visible, the series of dots are laid down at different angles, typically referred to as screen angles. Similar, but often less noticeable problems exist with this type of printing and can result in imperfections, such as banding or barring.
To address problems in the rotary printing industry, such as those problems mentioned above, it would be desirable to provide printing methods and apparatus which significantly reduce or prevent visible banding or barring on printed substrates.
In one aspect, the present invention provides a method of producing a vignette and preventing barring or banding during a rotary printing operation. The method includes moving a substrate in a linear direction or, in other words, in the machine direction with respect to a rotary printing member. The rotary printing member has an outer surface with a series of elongated printing dots configured to produce an image on the substrate. The dots have a length extending substantially in the linear direction, and the dots increase in density in a direction parallel to the linear direction. The dots have widths extending perpendicular to their lengths and the widths of the dots are substantially less than their respective lengths. Preferably, the width of each dot is between about 10% and about 50% of the length. The lengthwise extents of the dots preferably extend parallel to the machine direction, however, a deviation of about 10xc2x0 may be acceptable for certain applications. Preferably, only a single color is used in producing the vignette. The method further involves applying a printing medium, such as ink, to the elongated printing dots and transferring the printing medium from the rotary printing member to the substrate to produce the vignette.
Although the vignette may be produced in accordance with the invention using a number of different types of rotary printing apparatus, one preferred type of apparatus is a flexographic printing apparatus using at least one flexographic plate secured to a rotary printing cylinder. Using apparatus such as a flexographic printing press, the transfer of the printing medium is a direct transfer from the printing cylinder to the substrate, such as by impression against the printing cylinder using an impression cylinder. Other printing apparatus may indirectly transfer ink from a series of rotating members to the moving substrate. Other types of printing apparatus capable of use in accordance with the invention include rotogravure presses, letter presses, offset presses, FM screening presses and stochastic presses.
The invention further contemplates a rotary printing member for printing a vignette on a substrate, such as by using the method described above. The printing member may comprise a cylinder or other rotary printing member. In the specific case of a flexographic printing member, a flexible plate, which may be conventionally formed of a thermoplastic material, may be chemically etched according to conventional pre-press procedures to have the unique dot structure of the invention. The dots are formed as raised, elongated printing dots. In the preferred embodiment of the invention, the dots will have a length extending within about 10xc2x0 of the linear direction or machine direction as the printing medium is being transferred to the substrate. It has been found most preferred to have the unique dot structure of the invention exist in printed regions having a dot coverage or density of between about 40% and 50%. More generally, the problem of barring or banding appears to be most visible to the human eye in a density region between about 15% dot coverage and about 70% dot coverage.
The invention further contemplates a rotary printing member for printing a vignette on a substrate, such as by using the method described above. The printing member may comprise a cylinder or other rotary printing member. In the specific case of a flexographic printing member, a flexible plate, which may be conventionally formed of a thermoplastic material, may be chemically etched according to conventional pre-press procedures to have the unique dot structure of the invention. The dots are formed as raised, elongated printing dots. In the preferred embodiment of the invention, the dots will have a length extending within about 10xc2x0 of the linear direction or machine direction as the printing medium is being transferred to the substrate. It has been found most essential to have the unique dot structure of the invention exist in printed regions having a dot coverage or density of between about 40% and 50%. More generally, the problem of barring or banding appears to be most visible to the human eye in a density region between about 15% dot coverage and about 70% dot coverage.
The present invention further contemplates a rotary printing member for use in a process printing operation, such as the method described above. In accordance with the invention, and like the printing member used for producing a vignette, this printing member will have elongated dots extending in the machine direction or, in other words, perpendicular to the axis of rotation of the printing member. In the preferred embodiment, the length dimension of these dots extends parallel to the machine direction, however, a deviation of, for example, about 10xc2x0 may be acceptable for various applications as discussed above. In process printing, for example, it is important to have the darker colors, or more prominent colors, extending directly in the machine direction. Other, less noticeable colors may deviate from the machine direction. Especially for dark colors, a screen angle of more than 10xc2x0 should not result in dots having their length dimensions also angling at the screen angle. In other words, while a substantial screen angle of each set of dots may be used to prevent moirxc3xa9 patterns, the lengthwise dimensions of the elongated dots should still extend within the confines described above.
According to the invention, the dots are configured so that when skidding or relative movement takes place between the moving substrate and the rotating printing member or cylinder, a minimal amount of increased ink is laid down along the line of contact between the substrate and the print member. Thus, the barring or banding previously perceived becomes masked and invisible to the human eye. Printing an elongate dot, such as an elliptical dot, with the longest extent running in the machine direction reduces the amount of skid gain. That is, the area of ink along the dots located at the line of the skid is not increased nearly as much as when using conventional round dots. It is also generally desirable to chain link the elongated dots, also in the machine direction, to further prevent visible barring or banding in the printed subject matter.
These and other objects, advantages and features of the invention will become more readily apparent to those of ordinary skill in the art upon review of the following detailed description of the preferred embodiments taken in conjunction with the accompanying drawings.